Adherents argue that their beliefs are based on sensory perception and rationalism alone.

Members of the Flat Earth Society claim to believe the Earth is flat. Walking around on the planet's surface, it looks and feels flat, so they deem all evidence to the contrary, such as satellite photos of Earth as a sphere, to be fabrications of a "round Earth conspiracy" orchestrated by NASA and other government agencies.

The belief that the Earth is flat has been described as the ultimate conspiracy theory. According to the Flat Earth Society's leadership, its ranks have grown by 200 people (mostly Americans and Britons) per year since 2009. Judging by the exhaustive effort flat-earthers have invested in fleshing out the theory on their website, as well as the staunch defenses of their views they offer in media interviews and on Twitter, it would seem that these people genuinely believe the Earth is flat.

First, a brief tour of the worldview of a flat-earther: While writing off buckets of concrete evidence that Earth is spherical, they readily accept a laundry list of propositions that some would call ludicrous. The leading flat-earther theory holds that Earth is a disc with the Arctic Circle in the center and Antarctica, a 150-foot-tall wall of ice, around the rim. NASA employees, they say, guard this ice wall to prevent people from climbing over and falling off the disc. Earth's day and night cycle is explained by positing that the sun and moon are spheres measuring 32 miles (51 kilometers) that move in circles 3,000 miles (4,828 km) above the plane of the Earth. (Stars, they say, move in a plane 3,100 miles up.) Like spotlights, these celestial spheres illuminate different portions of the planet in a 24-hour cycle. Flat-earthers believe there must also be an invisible "antimoon" that obscures the moon during lunar eclipses.

Furthermore, Earth's gravity is an illusion, they say. Objects do not accelerate downward; instead, the disc of Earth accelerates upward at 32 feet per second squared (9.8 meters per second squared), driven up by a mysterious force called dark energy. Currently, there is disagreement among flat-earthers about whether or not Einstein's theory of relativity permits Earth to accelerate upward indefinitely without the planet eventually surpassing the speed of light. (Einstein's laws apparently still hold in this alternate version of reality.)

As for what lies underneath the disc of Earth, this is unknown, but most flat-earthers believe it is composed of "rocks."

Then, there's the conspiracy theory: Flat-earthers believe photos of the globe are photoshopped; GPS devices are rigged to make airplane pilots think they are flying in straight lines around a sphere when they are actually flying in circles above a disc. The motive for world governments' concealment of the true shape of the Earth has not been ascertained, but flat-earthers believe it is probably financial. "In a nutshell, it would logically cost much less to fake a space program than to actually have one, so those in on the Conspiracy profit from the funding NASA and other space agencies receive from the government," the flat-earther website's FAQ page explains.

It's no joke

The theory follows from a mode of thought called the "Zetetic Method," an alternative to the scientific method, developed by a 19th-century flat-earther, in which sensory observations reign supreme. "Broadly, the method places a lot of emphasis on reconciling empiricism and rationalism, and making logical deductions based on empirical data," Flat Earth Society vice president Michael Wilmore, an Irishman, told Life's Little Mysteries. In Zetetic astronomy, the perception that Earth is flat leads to the deduction that it must actually be flat; the antimoon, NASA conspiracy and all the rest of it are just rationalizations for how that might work in practice.

Those details make the flat-earthers' theory so elaborately absurd it sounds like a joke, but many of its supporters genuinely consider it a more plausible model of astronomy than the one found in textbooks. In short, they aren't kidding.

"The question of belief and sincerity is one that comes up a lot," Wilmore said. "If I had to guess, I would probably say that at least some of our members see the Flat Earth Society and Flat Earth Theory as a kind of epistemological exercise, whether as a critique of the scientific method or as a kind of 'solipsism for beginners.' There are also probably some who thought the certificate would be kind of funny to have on their wall. That being said, I know many members personally, and I am fully convinced of their belief."

Wilmore counts himself among the true believers. "My own convictions are a result of philosophical introspection and a considerable body of data that I have personally observed, and which I am still compiling,” he said.

Strangely, Wilmore and the society's president, a 35-year-old Virginia-born Londoner named Daniel Shenton, both think the evidence for global warming is strong, despite much of this evidence coming from satellite data gathered by NASA, the kingpin of the "round Earth conspiracy." They also accept evolution and most other mainstream tenets of science.

Conspiracy theory psychology

As inconceivable as their belief system seems, it doesn't really surprise experts. Karen Douglas, a psychologist at the University of Kent in the United Kingdom who studies the psychology of conspiracy theories, says flat-earthers' beliefs cohere with those of other conspiracy theorists she has studied.

"It seems to me that these people do generally believe that the Earth is flat. I'm not seeing anything that sounds as if they're just putting that idea out there for any other reason," Douglas told Life's Little Mysteries.

She said all conspiracy theories share a basic thrust: They present an alternative theory about an important issue or event, and construct an (often) vague explanation for why someone is covering up that "true" version of events. "One of the major points of appeal is that they explain a big event but often without going into details," she said. "A lot of the power lies in the fact that they are vague."

The self-assured way in which conspiracy theorists stick to their story imbues that story with special appeal. After all, flat-earthers are more adamant that the Earth is flat than most people are that the Earth is round (probably because the rest of us feel we have nothing to prove). "If you're faced with a minority viewpoint that is put forth in an intelligent, seemingly well-informed way, and when the proponents don't deviate from these strong opinions they have, they can be very influential. We call that minority influence," Douglas said.

In a recent study, Eric Oliver and Tom Wood, political scientists at the University of Chicago, found that about half of Americans endorse at least one conspiracy theory, from the notion that 9/11 was an inside job to the JFK conspiracy. "Many people are willing to believe many ideas that are directly in contradiction to a dominant cultural narrative," Oliver told Life's Little Mysteries. He says conspiratorial belief stems from a human tendency to perceive unseen forces at work, known as magical thinking.

However, flat-earthers don't fit entirely snugly in this general picture. Most conspiracy theorists adopt many fringe theories, even ones that contradict each other. Meanwhile, flat-earthers' only hang-up is the shape of the Earth. "If they were like other conspiracy theorists, they should be exhibiting a tendency toward a lot of magical thinking, such as believing in UFOs, ESP, ghosts, the Devil, or other unseen, intentional forces," Oliver wrote in an email. "It doesn't sound like they do, which makes them very anomalous relative to most Americans who believe in conspiracy theories."